The present invention is directed to an electroluminescent device; and, more particularly, to an organoelectroluminescent device having an improved luminous efficiency, good stability and prolonged lifetime, wherein the organic layer comprises a polyimide thin film having an emission and/or carrier transport capability.
Generally, an electroluminescent device has a laminated structure comprising a transparent electrode layer, a metallic electrode layer and an organic interlayer including an organic luminescent layer arranged between the two electrodes. The electroluminescent device can be operated with an alternate current(AC) or direct current(DC) power supply. In case of DC operation, the transparent electrode functions as an anode, and the metallic electrode as a cathode. In order to increase the luminous efficiency, the organic interlayer may further include hole transport agents and electron transport agents, often in a multilayer configuration.
For example, in case of DC operation, a separate hole transport layer may be disposed between, and in close contact with, the anode layer and one surface of the organic luminescent layer. Further, an optional electron transport layer may be placed between the cathode layer and the organic luminescent layer. Depending on the organic materials employed, therefore, the organic interlayer of an electroluminescent device may be in the form of a single, double or triple layers, each layer containing various combinations of organic luminescent materials, hole transport agents and electron transport agents. As the anode layer, indium tin oxide-glass layer is usually used while a metallic layer of magnesium, aluminum, indium or silver-magnesium can be used as the cathode. In conventional electroluminescent devices, the organic layers are usually formed by vapor-depositing the organic materials.
However, an organic layer, e.g., a hole transport layer, prepared by a conventional vacuum deposition method of small molecules has the disadvantages in that the deposited organic layer is fragile, easily broken by a vibrational shock, and the lifetime of the device is short due to the occurrence of crystallization and diffusion migration phenomena during use. In order to solve the above problems, an organic layer has been prepared by dispersing active agents, e.g., a hole transport agent and organic luminescent material, in a polymer matrix such as polymethylmethacrylate (PMMA) having a glass transition temperature of 105xc2x0 C., polycarbonate having a glass transition temperature of 145xc2x0 C., or other vinylic polymers having a glass transition temperature of 150xc2x0 C. or lower. However, these polymers have a low heat stability and thus the stability of the resultant organic layer is unsatisfactory(see Kido et al., Appl. Phys. Lett., 61, No. 7, 171, 1992; and Jpn. J. Appl. Phys., 31, No. 78, L960, 1992).
U.S. Pat. Nos. 5,609,970, 5,571,626, 5,414,069 and 5,376,456, on the other hand, disclose certain electroluminescent polymers. However, the preparation of these special polymers entails high production costs and complicated processing steps and thus are not suitable for the mass production of electroluminescent devices.
Polyimides have been widely used in the electronic and electrical fields. For example, it has been reported that polyimides may be used in packaging an electroluminescent device or as an insulating layer in electroluminescent devices (see U.S. Pat. Nos. 5,505,985 and 5,416,622). Further, Japanese Laid-Open Publication No. 4-93389 suggests that a polyimide film can be used as a carrier(hole) transport layer. However, the hole transport ability of a polyimide layer is too low for practical use. Japanese Laid-Open Publication No. 7-230881 discloses that a thermally decomposed silicon-containing polyimide exhibits a hole transporting ability, the thermally decomposed polyimide being prepared by heat treating a polyimide at a temperature ranging from 500xc2x0 C. to 1,000xc2x0 C. However, the turn-on voltage of the final device remains still too high.
Therefore, there has existed a need to develop a low-cost electroluminescent device having an improved stability and luminous efficiency.
Accordingly, it is an object of the present invention to provide an electroluminescent device having good thermal and mechanical properties, improved stability and prolonged lifetime.
In accordance with the present invention, there is provided an electroluminescent device comprising a transparent electrode layer, a metallic electrode layer, and an organic interlayer disposed between, and in close contact with, the electrode layers wherein the organic interlayer is comprised of an organic luminescent layer, an optional hole transport layer and an optional electron transport layer, and contains a polyimide of formula (I): 
wherein A is a moiety derived from a dianhydride compound; B is a moiety derived from a diamine compound; and n is an integer of 2 or higher.
In accordance with one aspect of the present invention, there is provided an electroluminescent device comprising a transparent anode layer, a metallic cathode layer, an organic luminescent layer containing an electroluminescent material and a hole transport layer, the organic luminescent layer being disposed between, and in close contact with, the cathode layer and one surface of the hole transport layer, and the other surface of the hole transport layer being in close contact with the anode layer, wherein the hole transport layer contains a hole transport agent dispersed in a polyimide matrix.
In accordance with another aspect of the present invention, there is provided an electroluminescent device comprising a transparent anode layer, a cathode layer and an organic luminescent layer containing an electroluminescent material, the electroluminescent layer being disposed between, and in close contact with, the anode and the cathode layers, wherein the electroluminescent material is dispersed in a polyimide matrix.
The above and other objects and features of the present invention will become apparent from the following description thereof, when taken in conjunction with the accompanying drawings wherein:
FIG. 1 shows a schematic diagram of an electroluminescent device in accordance with one embodiment of the present invention;
FIG. 2 demonstrates the variation of the current density(A/m2)(xe2x97xaf) and brightness(cd/m2) (∘) of the electro-luminescent device of Example 12 as a function of the applied voltage (V);
FIG. 3 exhibits the dependency of the luminous efficiency(lm/W) of the electroluminescent device of Example 12 on current density(A/m2); and
FIG. 4 depicts the change in light intensity of the electroluminescent device of Example 12 with time.